Question

Which of the following is a soft base?
(A) $N{{H}_{3}}$
(B) ${{R}_{2}}S$
(C) $C{{u}^{+}}$
(D) ${{H}_{2}}O$

Answer 1

Hint: To solve this question, we first need to understand what is a soft base. Acids or bases that have a low ratio between charge and radius is known as a soft acid or base. These are highly polarizable, have low electronegativity and usually have large ions.

Complete answer:
The concept of hard acids and bases comes from the Pearson acid-base concept. It is also known as the HSAB concept or the "hard and soft (Lewis) acids and bases".
It can be used to explain the reaction mechanism and pathways and the stability of a compound.
Now, in soft acids and bases, the atoms are larger, have low electronegativity and relatively polarizable. Like phosphorus, sulfur, and chlorine. Also, soft acids are the metal ions that have the highest affinity for a soft acid.
Whereas in hard acids and bases the atoms are small, have high electronegativity and relatively non-polarizable. Like nitrogen, oxygen, and fluorine. Also, hard acids are the metal ions that have the highest affinity for a hard base.
So, among the given options
(A) Ammonia ($N{{H}_{3}}$) is a hard base.
(B) Dialkyl sulfide (${{R}_{2}}S$) is a soft base.
(C) Copper cation ($C{{u}^{+}}$) is a soft acid.
(D) Water (${{H}_{2}}O$) is a hard base.

Hence the correct answer is option (B) ${{R}_{2}}S$ (dialkyl sulfide).

Note:
It should be noted that the
- Oxidation states of hard acids and bases are high whereas that of soft acids and bases is low or zero.
- Hard acids and bases have an affinity for ionic bonding whereas soft acids and bases have an affinity for covalent bonding.